A sealed secondary battery has been widely used as power sources for portable devices such as mobile phones and notebook computers, in which a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery or an aqueous electrolyte secondary battery such as a nickel-hydrogen secondary battery is sealed together with a protective circuit in a battery case such as a metal can or a laminate film. The protective circuit has a function for monitoring the voltage of the battery and blocking the charging current or discharging current when the overcharging or overdischarging occurs.
Since the non-aqueous electrolyte secondary battery uses a flammable organic electrolyte solution, the inside pressure of the battery may be raised by decomposition gas of the electrolyte solution and electrodes or vapor gas of the electrolyte solution generated by heat caused by overcharging or short circuit, and may cause the bulging of the battery, and may result in the explosion of the battery in the worst case. Moreover, in the case of aqueous electrolyte secondary battery, the battery may be expanded by the structural change of the electrode active material through charge and discharge and also by generated decomposition gas of the electrolyte solution and electrodes.
In contrast, a method for preventing rupture of the sealed secondary battery comprising detecting in advance the bulging of the sealed second battery and blocking the charging current or discharging current has been studied.
As a method of detecting the bulging of the sealed secondary battery, a method of using a PTC element is known (for example, Patent Document 1). The PTC element is electrically connected to the protective circuit, if a current exceeds the predetermined value and heat generation occurs in the sealed secondary battery, the element cuts off the current flowing between the sealed secondary battery and external devices by increasing the resistance value. However, as for the method of using the PTC element, there is a problem that the element cannot respond quickly to a rapid temperature rise of the sealed secondary battery. In contrast, methods for detecting bulging directly in place for detecting the heat generation caused by bulging have been proposed. According to this method, when the bulging occurs, it is possible to block current more quickly. For example, Patent Document 2 discloses a sealed secondary battery comprising a casing containing a positive electrode and a negative electrode and being capable of charging and discharging, and a safety element adhered to the surface of the casing and whose resistance value changes according to the bulging of the casing. It is described in Patent Document 2 that strain gauges are used as the safety element and those strain gauges are adhered to the surface of the casing using double-sided adhesive tape or double-sided adhesive film. Further, Patent Document 3 also discloses a battery in which strain gauges are adhered to the casing so as to detect the mechanical deformation of the battery.